Oximetry is the measurement of the oxygen status of blood. Early detection of low blood oxygen is critical in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of oxygen supply. A pulse oximeter typically provides a numerical readout of the patient""s oxygen saturation, a numerical readout of pulse rate, and an audible indicator or xe2x80x9cbeepxe2x80x9d that occurs in response to each pulse. In addition, a pulse oximeter may display the patient""s plethysmograph waveform, which is a visualization of blood volume change in the illuminated tissue caused by pulsatile arterial blood flow over time. The plethysmograph provides a visual display that is also indicative of the patient""s pulse and pulse rate.
A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor. Conventionally, a pulse oximetry sensor has both red and infrared (IR) light-emitting diode (LED) emitters and a photodiode detector. The sensor is typically attached to a patient""s finger or toe, or a very young patient""s patient""s foot. For a finger, the sensor is configured so that the emitters project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode is positioned at the fingertip opposite the fingernail so as to detect the LED transmitted light as it emerges from the finger tissues.
The pulse oximetry monitor (pulse oximeter) determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor. The pulse oximeter alternately activates the sensor LED emitters and reads the resulting current generated by the photodiode detector. This current is proportional to the intensity of the detected light. The pulse oximeter calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The pulse oximeter contains circuitry for controlling the sensor, processing the sensor signals and displaying the patient""s oxygen saturation and pulse rate. A pulse oximeter is described in U.S. Pat. No. 5,632,272 assigned to the assignee of the present invention.
FIG. 1 illustrates the standard plethysmograph waveform 100, which can be derived from a pulse oximeter. The waveform 100 is a display of blood volume, shown along the y-axis 110, over time, shown along the x-axis 120. The shape of the plethysmograph waveform 100 is a function of physiological conditions including heart stroke volume, pressure gradient, arterial elasticity and peripheral resistance. The ideal waveform 100 displays a broad peripheral flow curve, with a short, steep inflow phase 130 followed by a 3 to 4 times longer outflow phase 140. The inflow phase 130 is the result of tissue distention by the rapid blood volume inflow during ventricular systole. During the outflow phase 140, blood flow continues into the vascular bed during diastole. The end diastolic baseline 150 indicates the minimum basal tissue perfusion. During the outflow phase 140 is a dicrotic notch 160, the nature of which is disputed. Classically, the dicrotic notch 160 is attributed to closure of the aortic valve at the end of ventricular systole. However, it may also be the result of reflection from the periphery of an initial, fast propagating, pressure pulse that occurs upon the opening of the aortic valve and that precedes the arterial flow wave. A double dicrotic notch can sometimes be observed, although its explanation is obscure, possibly the result of reflections reaching the sensor at different times.
FIGS. 2-4 illustrate plethysmograph waveforms 200, 310, 360 that display various anomalies. In FIG. 2, the waveform 200 displays two arrhythmias 210, 220. In FIG. 3, the waveform 310 illustrates distortion corrupting a conventional plethysmograph 100 (FIG. 1). FIG. 4 shows a filtered waveform 360 after distortion has been removed through adaptive filtering, such as described in U.S. Pat. No. 5,632,272 cited above. FIG. 4 illustrates that, although the waveform 360 is filtered, the resulting pulses 362 have shapes that are distorted in comparison to the pulses illustrated in FIG. 1.
A desirable feature of pulse oximeters is an audible xe2x80x9cbeepxe2x80x9d tone produced to correspond to the patient""s pulse. Conventionally, the beep is triggered from recognition of some aspect of the plethysmograph waveform shape. Such a waveform-triggered beep may indicate an arrhythmia, like those displayed in FIG. 2, but may also generate false pulse indications as the result of motion-artifact or noise induced waveform distortion, as illustrated in FIGS. 3 and 4. This characteristic results because both distortion and arrhythmias result in anomalies in the plethysmograph waveform shape on which this beep mechanism is dependent. Alternatively, the beep can be triggered from a time base set to the average pulse rate. Signal processing can generate an average pulse rate that is resistant to distortion induced error. A pulse beep based on average pulse rate is relatively insensitive to episodes of distortion, but is likewise insensitive to arrhythmias.
An example of the determination of pulse rate in the presence of distortion is described in U.S. Pat. No. 6,002,952, filed Apr. 14, 1997, entitled xe2x80x9cSignal Processing Apparatus and Method,xe2x80x9d which is assigned to the assignee of the current application and incorporated by reference herein. Another example of pulse rate determination in the presence of distortion is described in U.S. patent application Ser. No. 09/471,510, filed Dec. 23, 1999, entitled xe2x80x9cPlethysmograph Pulse Recognition Processor,xe2x80x9d which is assigned to the assignee of the current application and incorporated by reference herein.
One aspect of the present invention is a processor having a decision element that determines if the waveform has little or no distortion or significant distortion. If there is little distortion, the decision element provides a trigger in real-time with physiologically acceptable pulses recognized by a waveform analyzer. If there is significant distortion, then the decision element provides the trigger based synchronized to an averaged pulse rate, provided waveform pulses are detected. The trigger can be used to generate an audible pulse beep that is insensitive to episodes of significant distortion, but is capable of responding to arrhythmia events.
Another desirable feature for pulse oximeters is a visual indication of the patient""s pulse. Conventionally, this is provided by an amplitude-versus-time display of the plethysmograph waveform, such as illustrated in FIG. 1. Some monitors are only capable of a light-bar display of the plethysmograph amplitude. Regardless, both types of displays provide a sufficient indication of the patient""s pulse only when there is relatively small distortion of the plethysmograph waveform. When there is significant distortion, such as illustrated in FIG. 3A, the display provides practically no information regarding the patient""s pulse.
Yet another desirable feature for pulse oximeters is an indication of confidence in the input data. Conventionally, a visual display of a plethysmograph waveform that shows relatively small distortion would convey a high confidence level in the input data and a corresponding high confidence in the saturation and pulse rate outputs of the pulse oximeter. However, a distorted waveform does not necessarily indicate low confidence in the input data and resulting saturation and pulse rate outputs, especially if the pulse oximeter is designed to function in the presence of motion-artifact.
Another aspect of the current invention is the generation of a data integrity indicator that is used in conjunction with the decision element trigger referenced above to create a visual pulse indicator. The visual pulse indicator is an amplitude-versus-time display that can be provided in conjunction with the plethysmograph waveform display. The trigger is used to generate a amplitude spike synchronous to a plethysmograph pulse. The data integrity indicator varies the amplitude of the spike in proportion to confidence in the measured values.
Yet another aspect of the present invention is a processing apparatus that has as an input a plethysmograph waveform containing a plurality of pulses. The processor generates a trigger synchronous with the occurrence of the pulses. The processor includes a waveform analyzer having the waveform as an input and responsive to the shape of the pulses. The processor also includes a decision element responsive to the waveform analyzer output when the waveform is substantially undistorted and responsive to pulse rate when the waveform is substantially distorted. The trigger can be used to generate an audible or visual indicator of pulse occurrence. A measure of data integrity can also be used to vary the audible or visual indicators to provide a simultaneous indication of confidence in measured values, such as oxygen saturation and pulse rate.
A further aspect of the current invention is a method of indicating a pulse in a plethysmograph waveform. The method includes the steps of deriving a measure of distortion in the waveform, establishing a trigger criterion dependent on that measure, determining whether the trigger criterion is satisfied to provide a trigger, and generating a pulse indication upon occurrence of the trigger. The deriving step includes the substeps of computing a first value related to the waveform integrity, computing a second value related to the recognizable pulses in the waveform, and combining the first and second values to derive the distortion measure. The trigger criterion is based on waveform shape and possibly on an averaged pulse rate.
One more aspect of the current invention is an apparatus for indicating the occurrence of pulses in a plethysmograph waveform. This apparatus includes a waveform analyzer means for recognizing a physiological pulse in the waveform. Also included is a detector means for determining a measure of distortion in the waveform and a decision means for triggering an audible or visual pulse indicator. The decision means is based the physiological pulse and possibly the pulse rate, depending on the distortion measure.
Another aspect of the present invention is a data confidence indicator comprising a plurality of physiological data and a plurality of signal quality measures derived from a physiological sensor output. A plurality of comparator outputs are each responsive to one of the measures and a corresponding one of a plurality of thresholds. An alert trigger output combines said comparator outputs, and a low signal quality warning is generated in response to said alert trigger output. The thresholds are set so that the warning occurs during a time period when there is low confidence in the data. In one embodiment, the warning is a display message that supplements a visual pulse indicator, the display message specifies a low signal quality when the visual pulse indicator has an amplitude that is less than one-third full-scale. In another embodiment, the signal quality measures are an integrity measure, a pulse rate density measure and a harmonic ratio measure. In a particular embodiment, the thresholds may have an integrity value of less than 0.3, a pulse rate density value of less than 0.7 and a harmonic ratio value of less than 0.8.
In yet another embodiment a filter for the data generates a smoothed data output. An adjustment for the smoothed data output is a function of at least one of the signal quality measures so that smoothing at the smoothed data output increases when at least one of the signal quality measures decreases. An alarm trigger is responsive to the smoothed data output so as to generate an alarm when the smoothed data output is outside of a predetermined limit. In a particular embodiment the filter comprises a buffer having a buffer input and a delay output. The buffer input corresponds to the data and the delay output is time-shifted according to the adjustment. A first filter comparator output is responsive to the data and a data threshold, and a second filter comparator output is responsive to the delay output and a delay output threshold. The comparator outputs are combined so as to provide the alarm trigger.
A further aspect of the present invention is a data confidence indicator comprising a processor configured to derive a time-dependent physiological data set and a plurality of time-dependent signal quality measures from a physiological signal. A buffer is configured to time-shift the data set by a delay to generate a delayed data set, where the delay is a function of at least one of the signal quality measures. The indicator has a threshold setting a limit for the delayed data set. A warning is generated when the levels of the data set and the delayed data set are beyond that threshold. In one embodiment, a first comparator output is responsive to the data and the threshold, and a second comparator output is responsive to the delayed data set and the threshold. A combination of the first and second comparator outputs provides an alarm trigger for the warning. The data confidence indicator may also comprise a combination of the signal quality measures providing an alert trigger to generate warning when confidence in the data set is low.
An additional aspect of the present invention is a data confidence indication method comprising the steps of acquiring a signal from a physiological sensor, calculating a physiological data set from the signal, calculating signal quality measures from the signal, and indicating on a display the confidence in the data set based upon at least one of the signal quality measures. The indicating step may have the substeps of utilizing the signal quality measures to detect a low signal quality period during which time the data set may be compromised, and writing an alert message on the display during at least a portion of that period. Additional utilizing substeps may include comparing each of the signal quality measures to a corresponding one of a plurality of thresholds to generate a plurality of trigger inputs and combining the trigger inputs to trigger a low signal quality warning. Additional steps may include setting an alarm limit for the data set, filtering the data set to generate an alarm trigger based upon the alarm limit and adjusting the characteristics of the filtering step according to at least one of the signal quality measures so that more filtering is applied during the low signal quality period. In one embodiment, the filtering step comprises the substeps of time-shifting the data set to create a delayed data set, comparing the data set to a threshold to generate a first trigger input, comparing the delayed data set to the threshold to generate a second trigger input, and combining the trigger inputs to generate the alarm trigger.
Yet a further aspect of the present invention is a data confidence indication method comprising the steps of acquiring a signal from a physiological sensor, calculating a physiological data set from the signal, calculating a plurality of signal quality measures from the signal, setting an alarm threshold for the data set, and delaying an alarm trigger when the data set exceeds the threshold as a function of at least one of the signal quality measures so as to reduce the probability of false alarms. In one embodiment, the delaying step comprises the substeps of time-shifting the data set by a delay to generate a delayed data set, where the delay is a function of at least one of said signal quality measures, and comparing the data set to the threshold to create a first limit output. Further substeps include comparing the delayed data set to the threshold to create a second limit output and combining the limit outputs to generate the alarm trigger. The data confidence indication method may further comprise the steps of comparing each of the signal quality measures to a corresponding one of a plurality of thresholds to generate a plurality of trigger inputs and combining the trigger inputs to trigger a low signal quality warning.